Since its inception, the aeronautical industry has advanced in its research towards obtaining materials with certain properties that allow the correct operation of aircraft throughout their operating range.
Several characteristics are looked for in an airworthy material, such as its mechanical and fatigue strength, elasticity, density, and resistance to corrosion, as well as its price.
Research into new materials and their use plays an important role in the aerospace industry. There is still plenty of scope for optimising their use and, above all, for introducing nanomaterials into aircraft construction.
A good indicator of the suitability of an aeronautical material is its mechanical strength/density ratio, which is known as its specific strength, and which should be high in any material of interest.
In addition, we have to keep in mind that these properties must be preserved in all the working conditions of an aircraft. This includes airport runways, where aircraft can encounter 40 ºC in the summer and high humidity conditions, as well as cruise conditions, at altitudes of around 11,000 m, where the temperature drops to -50 ºC.
Another factor to consider is that during operation structural elements are subjected to drastic changes in load distribution. For example, wing sockets work by “supporting” the wings while the aircraft is on the ground. Then, while in flight, they “support” the fuselage, due to the lift generated by the wings. Thus, the traction and compression zones are reversed.
We also have to take into account the vibrations experience by the aircraft while in the air. This was a problem when the first regular flights began to take place, since the materials used had not been selected according to their fatigue strength. Some aircraft therefore began to collapse after a few years of sound operation.
As the industry has evolved in terms of its materials, two of the major research foci have been the increase of specific strength of materials and the improvement of the manufacturability of aircraft and their components.
Aviation took its first steps with models made of wood and textiles. Wood fulfilled the structural function, while the fabric provided the means to achieve lift. Despite its low density, some kinds of wood are fairly strong; however, this material is affected by biological action, in addition to reacting negatively to moisture.
It quickly gave way to the use of metal for the aircraft structure, specifically steel. However, steel’s high density, low resistance to corrosion and the galvanic couple it forms with aluminium ruled it out as a good airworthy material. Nowadays, its use in aeronautics has been relegated to very specific parts, such as the landing gear and certain hardware.
The next step was aluminium. In truth, in its pure state aluminium does not have good mechanical properties. Moreover, until a few decades ago it was a very expensive metal. However, advances in the process of obtaining aluminium and the use of aluminium alloys led to its rise as an aeronautical material.
Today the predominant alloys in the industry are known as duralumins. These types of alloys offer greater specific strength than steel, and also improve on other properties. Some of these aeronautical alloys belong to the 2XXX series, which is aluminum-copper, and the 7XXX series, aluminium-zinc. Nowadays, progress is being made in new alloys, such as aluminium-lithium alloys.
Another advantage of aluminium is its corrosion behaviour. This is thanks to its oxide, alumina, which when formed completely covers the base metal, leaving its surface protected in a process called “passivation”.
In certain parts of the aircraft, such as the engine, it is necessary to use alloys that have good thermal resistance, which generally involves the use of titanium alloys. The density of titanium is still lower than that of steel, although it is higher than that of aluminium. It has high corrosion resistance (it is also passive) and maintains good mechanical properties. The main drawbacks of titanium are its high cost and the difficulty it presents for machining. For this reason, it is only used in some parts of the engine or as a covering for hypersonic aircraft, where the interaction with shock waves generates high temperatures. The most commonly used alloy is Ti-6Al-4V.
In recent years, non-metallic materials have become increasingly important in aircraft construction, especially composite materials. These materials arise from the union of two or more different materials that are not soluble in each other and that are separable by mechanical action. One of the materials acts as a matrix and the other acts as the reinforcement. The matrix is responsible for giving shape and cohesion to the composite material, as well as transmitting compressive forces, while the reinforcement improves its behaviour with regards to the other mechanical loads. The most commonly used composites in aeronautics are those that use carbon fibre or fibreglass as the reinforcement.
Composite materials have considerably increased their presence in aeronautics, making up 50% of the last two models produced by large aircraft manufacturers: the Airbus A350 XWB and the Boeing B787 Dreamliner.
In addition, these materials allow better integration of parts due to their forming. An example of this is the construction of the one-piece tail cone of the A350 XWB. This strategy reduces costs by decreasing the number of transfers of pieces, the number of subsequent assemblies and the added weight due to joints.
Another factor to consider is the environmental side, in an industry in which a commitment to the environment is becoming increasingly important. The future forecasts of large manufacturers predict the use of environmentally friendly compounds and biopolymers, which would facilitate recycling.
But if there is one branch of research that should be highlighted for its novelty, it is the use of nanomaterials in aircraft construction. The use of nanomaterials aims to achieve structural and functional improvements, in addition to weight reduction.
Over the next few years, companies that wish to achieve and maintain their position as leading firms are set to continue researching new alternatives. Some of the advances that the industry predicts in this field are based on the search for self-healing materials or materials that adapt their shape to passengers.
In any case, this will only be achieved through greater investment, both in the initial design of the product and in its production process. The motivation behind this investment must focus on the improvements that will be produced in all the productive aspects mentioned above, in order to develop a more efficient, modern and clean industry.